Puccini's "Tosca" is about the collision of primal forces: love, jealousy, the brutal power of church and state. Pretty much any production is packed with drama. But Thursday's San Francisco Opera opening delivered more than anyone expected, when superstar soprano Angela Gheorghiu -- singing the lead role of Floria Tosca, the doomed diva -- withdrew after the first act at War Memorial Opera House.

Baseball has its relief pitchers, and opera has its understudies, known as "covers." So when Gheorghiu was taken ill with "a serious intestinal flu attack" -- David Gockley, the company's general director, announced the switch at the first intermission --- Moore was called into action. She spent about 20 minutes getting costumed, while the audience waited.

Then the soprano came out and blew the doors off this production with her presence, her passion, her elegance. Holy smokes. After she sang "Vissi d'arte" --- the heart-breaking aria, the most famous tune in Puccini's best opera -- the audience burst into sustained applause. Conductor Nicola Luisotti stood in the pit, beaming at Moore, wagging his baton in her direction, as if to say, "You, you, you!"

Incredibly, Moore had never before performed this demanding role, anywhere.

But from the moment she walked onstage in the second act to confront Scarpia -- chief of police in the Roman police state, circa 1800 --- she was in command.

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She was volatile, majestic yet earthy, ranging through her voice's many colors and effects: opalescent, smoky, deeply lustrous. She is an instinctive actor, too, conveying as much through gesture and facial expression as through her singing.Moore made this Tosca a real human being -- an artist, a lover, a schemer, and, yes, a killer. She will do whatever it takes to save the painter Mario Cavaradossi, her man -- who is tortured in Scarpia's basement throughout most of this second act.

This sort of walk-on magic can make a career.

Not that Moore is an unknown; no, her talents are widely recognized. She came up through the company's Merola and Adler Fellowship programs. She has sung big main-stage San Francisco roles before, including Mimi in Puccini's "La bohème" and Susan Rescoria in "Heart of a Soldier," the 9/11-themed opera by composer Christopher Theofanidis.

Still, to deliver so thrillingly in such a pressure situation as this -- it could (and should) turn Moore into a name-brand soprano for companies far and wide. (In the spirit of full disclosure, I'll mention that one or two of Moore's high notes were slightly insecure. But it almost increased the humanity of her performance -- and, besides, she hadn't had time to warm up. It didn't take long.)

Oh, I almost forgot to tell you about the rest of this production. It's gorgeous: Conceived by Lotfi Mansouri in 1997 and designed by Thierry Bosquet, it is a re-creation of the "Tosca" that opened War Memorial Opera House 80 years ago, in 1932. Taking us into the church of Sant'Andrea della Valle, the Act 1 sets are especially vast and opulent -- something of an extension beyond the proscenium arch of War Memorial Opera House itself.

Directed by Jose Maria Condemi, the production is stately and sure-footed, moving toward its inevitable conclusion, which always feels like a blow, no matter how many times you've seen "Tosca." Luisotti and the orchestra channeled Puccini's sadness and cynicism; the low brass played with frightening expression. And the singing by the opening night cast -- the first of two casts in rotation-- was outstanding.

As Scarpia, Italian baritone Roberto Frontali truly was the big bad wolf, with his bone crushing voice and chilling presence; memorable. Debuting with the company, Italian tenor Massimo Giordano was a charismatic Cavaradossi, youthful and fresh: his desperate final aria, "E lucevan le stelle," was honeyed yet virile, tree-trunk strong, top to bottom. As Cesare Angelotti, freedom fighter and Mario's pal, American bass-baritone Christian Van Horn is tall as an NBA forward; he: sang with towering force and fine-grained tone.

American bass-baritone Dale Travis was the Sacristan, ripe-voiced and likable, even though he folds before Scarpia. Second-year Adler Fellow Ryan Kuster, another American bass-baritone, was sonorous and poised as the Jailer.

And I suppose you want to know about Gheorghiu, the Romanian soprano around whom we all expected this night to revolve? She entered the first act dressed like a virgin bride in a long white gown -- and sang somewhat chastely during Tosca's famous love duet with Cavaradossi. Her voice was jewel-like, beautiful, but light. She wasn't projecting; she seemed at a remove. And while she grew more eruptive later in the act, this wasn't the Gheorghiu everyone expected.

Funny how these things work out. The "news" of this "Tosca" was supposed to lie in its star-laden casts, each led by a box-office soprano: Gheorghiu and Patricia Racette. Instead, this opening night at the opera gave us Moore's unscheduled, red-blooded performance. The libretto inadvertently played into the scenario at the start of the second act, when Scarpia sings (in Italian): "The diva is still missing from the concert." The audience laughed, but while Gheorghiu was missing, Moore was about to arrive.

Incidentally, word has it that Gheorghiu is expected to return to the production Sunday, for her next scheduled performance. I'm glad she's feeling better. I'm also glad to have seen the awesome Melody Moore.

By Giacomo Puccini, with libretto by Luigi Illica and Giuseppe Giacosa, presented by San Francisco Opera

Through: Dec. 2, with two casts in rotationWhere: War Memorial Opera House, 301 Van Ness Ave., San FranciscoTickets: $22-$340, 415-864-3330, www.sfopera.com ($10 standing room tickets go on sale at 10 a.m. on the day of each performance)